1. Technical Field
The present invention relates to a configuration of a hybrid vehicle mounted with an engine, an electric motor, and a power generator.
2. Related Art
Recently, hybrid vehicles mounted with an engine, an electric motor, and a power generator are widely used in various methods, including where a hybrid vehicle is driven by the combination of an engine output and an electric motor output depending on running conditions; and where while charging a battery by driving an electric motor with some of the engine output, a vehicle is driven by the combination of the remaining engine output and the electric motor output; and further where by driving a power generator with engine output, the generated power is used to drive an electric motor to drive a vehicle. In many cases of such a hybrid vehicle, DC low voltage of a battery is boosted to DC high voltage by a boost converter, and the boosted voltage is supplied to an electric motor or a power generator and further to each of inverters which transfer electric power to or from the electric motor or the power generator, and the electric motor is driven by converting the DC power to three-phase AC power used to drive the electric motor by each inverter, or the three-phase AC power generated by the power generator is converted to DC power.
The boost converter is used to turn ON or OFF a switching device and to boost DC low voltage of a battery by using stored energy in a reactor to output DC high voltage. Accordingly, a boost loss occurs from the ON and OFF operation of the switching device. The boost loss increases along with the increase in output electric power and boost ratio (ratio of the DC high voltage to the DC low voltage) of the boost converter, while the boost loss decreases along with the decrease of the output voltage and the boost ratio. Even when the output power of the boost converter is zero, indicating a no load state, the boost loss (switching loss) does not decrease down to zero as long as the switching device continues to be turned ON and OFF.
Ina hybrid vehicle, when the vehicle is running, for example, with the electric power generated by the power generator being balanced with the electric power consumed by the electric motor, because the electric motor can be driven by the electric power generated by the power generator without supplying the DC high voltage obtained by boosting the DC low voltage of a battery, the vehicle can continue running while maintaining the DC high voltage of the inverter at the current state. In this case, as the boost converter has no load, it may appear to be possible to improve the system efficiency of the hybrid vehicle by stopping the operation of the boost converter to reduce the boost loss (switching loss). However, because it is unlikely that the electric power consumed by the electric motor and the electric power generated by the power generator are completely balanced, if the boost converter is stopped when, for example, the electric power consumed by the electric motor is slightly larger than the electric power generated by the power generator, the DC high voltage of the inverter will gradually decrease. Therefore, methods are proposed to maintain the DC high voltage of the inverter at a target voltage by stopping the operation of the boost converter and by correcting the output torque of the electric motor so as to maintain the output power of the power generator at a constant level to minimize the deviation of the DC high voltage of the inverter from the target voltage when the electric power generated by the power generator and the electric power consumed by the electric motor are substantially balanced (refer to, for example, JP 2011-15603 A).